First Degree Heart Block


PR interval > 200ms (five small squares)

  • There is delay, without interruption, in conduction from atria to ventricles
  • ‘Marked’ first degree heart block is present if PR interval > 300ms
ECG Rhythm strip PR interval prolonged 1st degree AV block
First degree heart block

Causes of First Degree Heart Block
Clinical significance
  • As an isolated finding this is a benign entity that does not cause haemodynamic instability
  • No specific treatment is required
ECG Examples
Example 1
ECG Rhythm strip PR interval prolonged extreme 1st degree AV block
  • Marked first degree heart block
  • PR interval > 300 ms, P waves are buried in the preceding T wave

Example 2
ECG 1st degree AV block 1
  • Sinus bradycardia with 1st degree AV block
  • PR interval > 300 ms

Example 3
ECG 1st degree AV block 2
  • Normal sinus rhythm with 1st degree AV block
  • PR interval 260 ms

Advanced Reading



LITFL Further Reading


Emergency Medicine Specialist MBChB FRCEM FACEM. Medical Education, Cardiology and Web Based Resources | @jjlarkin78 | LinkedIn |

MBBS (UWA) CCPU (RCE, Biliary, DVT, E-FAST, AAA) Adult/Paediatric Emergency Medicine Advanced Trainee in Melbourne, Australia. Special interests in diagnostic and procedural ultrasound, medical education, and ECG interpretation. Editor-in-chief of the LITFL ECG Library. Twitter: @rob_buttner


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